The identifying call letters of the various radio navigational aids used in aircraft and marine transportation are transmitted by means of tone modulated Morse code. These identifying code groups are transmitted at a speed of approximately seven words per minute with a repetition rate of between two and twenty times per minute as allowed by the International Civil Aviation Organization (ICAO) Aeronautical Telecommunications, Annex 10.
The prior state of the art required that the pilot or crew member listen to and identify the Morse code dots and dashes while concurrently performing other piloting functions as well. Often the signal received by the navigation equipment is very noisy causing many pilots to neglect this very important identification task altogether. Such inadequate piloting procedures have been cited by the National Transportation and Safety Board (NTSB) as a contributing factor in several aircraft accidents.